It is not uncommon for your iliacus to go into spasm in reaction to some sort of back stress or trauma. This can be quite painful, because it will cause a sympathetic reaction in other muscle groups in your back and abdomen.

The tricky thing is that it's impossible to palpate your iliacus because it is completely inside your pelvis. If it is in spasm, your best bet is to apply pressure to a trigger point of a related muscle. To do this: First, lay on your back and elevate your knees, and try to release as much tension from your abdomen as possible. Put your hand on the top of the front of your pelvis bone, on either side. Slide your thumb along the inside ridge of the bone until you just pass over that small bump on the crest of your pelvis. Now press inwards with your thumb so you start to feel the inside of the front of the pelvis. Apply steady pressure here, but not enough to cause any more pain. You should start to feel some release deep inside your iliacus, on the side on which you're working. Hold for a while, and then repeat the whole procedure for the other side. If you can, try to remain laying down with your knees elevated for a little while after you do this.